This invention is an improvement over the apparatus and method disclosed in U.S. Pat. No. 3,798,882. Commercial embodiments of the apparatus disclosed in said patent fail to attain the expected efficiency. Extensive effort to adjust and/or modify the apparatus disclosed in said patent was unproductive. The only manner in which it was possible to ascertain why the apparatus in said patent was not attaining the expected efficiency was by constructing the entire apparatus from transparent polymeric plastic material and observation of the filter bed during filtering and backwash.
The observation referred to above revealed two significant features which contributed to the lack of attaining the expected efficiencies. First, it was ascertained that fluidization of the beds during backwashing could not be accomplished by pulsed jets. As disclosed in said patent, backwashing involves high pressure indicated as preferably between 150 and 200 psi accomplished by two jets or pulses of air timed so as to occur approximately 0.10 to 0.15 seconds delay. It has now been ascertained that the use of pulses causes the bed to rise like a piston and descend like a piston thereby blocking flow through the screen and failing to attain the fluidized state.
A second significant point which contributed to the failure to attain the fluidized state was the ratio of the size of the filter chamber in each stack as compared with the height of the filter bed. As stated in said U.S. Pat. No. 3,798,882, approximately 25% of the space in the filter chambers above the filter beds is unoccupied so that the bed may be fluidized into that space. I have now ascertained that an unoccupied space of 25% is significantly inadequate. I have found that the unoccupied space above the filter bed must be at least 60% of the chamber volume and may be as high as 90%.
Other changes in the apparatus of the present invention as compared with the apparatus in said patent include minimizing pressure drop during filtration. As a result of the above-mentioned changes and other changes to be described in greater detail hereinafter, the apparatus of the present invention now attains the anticipated efficiencies that were not heretofore attained by the apparatus in said U.S. Pat. No. 3,798,882.